Numerous detection and analysis methods for determining physiological parameters in body fluid samples or other biological samples are carried out in an automated fashion in large numbers in corresponding automatic analysis devices. Modern analysis devices are capable of carrying out a multiplicity of detection reactions and analyses with a sample. In order to be able to carry out a multiplicity of examinations in an automated fashion, it is necessary to transfer small quantities of liquid to a multiplicity of positions by automated pipetting. For example, aliquots of the sample liquids must be taken from the sample vessels or accurately predetermined subquantities of reagents must be taken from the reagent containers and transferred into the reaction vessel intended for the examination in question. To this end, depending on the intended purpose, a plurality of corresponding pipetting systems is provided in the analysis device.
Such a pipetting system conventionally comprises a pipetting needle on an actively mobile element, for example a transport arm, the pipetting needle being fastened to a needle holder on the transport arm. The pipetting needle is configured as a hollow needle, which can take and deliver defined quantities of sample in an automated fashion. The pipetting needle is conventionally introduced along the mid-axis of a vessel, where appropriate piercing of a resilient stopper may occur in the case of closed vessels, and the pipetting needle is immersed into the liquid. The immersion is registered by means of a corresponding sensor, and the intended quantity of liquid is aspirated with controlled pressure. The quantity taken is then supplied to the corresponding analysis. The pipetting needle is subsequently washed in a corresponding device and is ready for its next use.
If an error in terms of positioning occurs during operation of the pipetting system, unintended contact, i.e., a crash, may take place. Even slight contact can cause errors in the immersion registration. Because of the sometimes high forces acting, however, damage to the needle or the object touched may also occur, or unintended transfer of sample material and therefore vitiation of the test results may take place. It is therefore absolutely necessary to avoid such crashes. Precisely because of the small diameters of the openings of the vessels used, namely a few millimeters, this can be ensured only by exact adjustment of the vessel positions and of the pipetting needle.
One problem is that the pipetting needle constitutes a disposable part, which regularly needs to be replaced. In particular, the frequent piercing of vessel stoppers leads to metal abrasion and bending of the needle. When the needle is replaced, however, the new needle tip has a different position in space, and the risk arises that the needle tip will depart so far from the setpoint position that the small openings of the vessels will be missed. Even a deviation of a few millimeters is sufficient for this.
To date, therefore, replacement of a pipetting needle has typically entailed comprehensive readjustment. With gauges, the new pipetting needle is fitted by hand and the adjustment is checked by comprehensive tests. Adjustable needles, which are fastened by means of wedge bearings, have been developed for this. A wedge bearing is respectively fitted to the needle on the right and on the left, the wedge of the needle having a smaller wedge angle than the counterbearing wedge of the needle holder. There is therefore only line contact on the right and on the left. This on the one hand cannot be measured accurately heightwise, and on the other hand it still allows a tilting movement of the needle. This tilting movement is spatially adjusted by means of setscrews. After adjustment, the setscrews are sealed with screw lacquer. Sometimes, pipetting middles are also adjusted by eye, struts of the frame or other straight housing parts being used for orientation. The vertical should in this case always be determined in two directions per needle. Such a method, however, is on the one hand error-prone and on the other hand expensive, since it typically cannot be carried out by the user himself; instead the replacement of the pipetting needle must be performed by a specially trained service engineer. Furthermore, the bearing is not especially stable and cannot withstand large forces, so that the needle slipping out during operation is not ruled out, and this makes even more frequent adjustment necessary.